Digital data may be stored on flexible or "floppy" disks or on hard or Winchester-type disks by the magnetization of successive small areas on the magnetic surface of the disk, by a magnetic head or "slider", as the disk rotates. The density of digital storage on hard disk memory systems is in the order of 10 or 20 times the density achieved with floppy disk memory systems.
Digital data processing systems usually operate in the binary system rather than in the decimal system, with numbers being represented by binary digits or "bits," usually represented by "1's" and "0's." Groups of eight bits of binary information are referred to as "bytes." Hard disk drives of the nominal 51/4 inch disk size vary in their storage capacity, with some being capable of storing more than a billion bytes of digital information.
Typically the surfaces of hard disks are divided into circular tracks. Tracks are further subdivided into sectors. A sector comprises a predetermined portion of the arc of a track. A fairly recent development is grouping tracks into several zones, each with a different data recording frequency in order to maximize data storage capacity. Typically, tracks located in zones further from the center of the disk have more sectors than tracks located in zones closer to the center, due to a higher data frequency at the outer radius. Though each data-bearing sector within a zone is the same size, the last partial sector on a track is typically a "runt" or incomplete sector which is not used to store data. The computer system, specifically the hard disk drive assembly portion of the computer system, uses the concept of tracks, sectors and zones as a means of defining data storage areas on the disks.
The hard disk drive assembly must generate a "sector pulse" signal at the beginning of each data-bearing sector in order to trigger the drive controller's data sequencer.
One prior art approach to generating sector pulses in shown in FIG. 1. A register 21 is used to store the number of bytes in a sector. That value is pre-loaded into the counter 23. The counter 23 counts down in synchronism with the servo-clock which generates one pulse per byte. The counter generates a signal T-count when it reaches zero which causes the sector generator 25 to generate a sector pulse. Also, when the counter 23 reaches zero, it reloads to the value it had earlier received from register 21 and again begins counting down. The prior art system shown in FIG. 1 is unable to take full advantage of the new concept of dividing disk surfaces into zones with different zones having different sector sizes and a different number of sectors, because when a head switches from a track in one zone to a new track in a new zone, the system cannot determine the sector start locations until one revolution is completed and the index, marking the beginning of the track, is passed.
Some currently existing systems which are utilized with the concept of zones employ what is called "soft sectoring." A soft sectoring system includes sector marks encoded directly on the data surface of the disks for each head. However, this causes two problems. First, encoding the information onto the disk itself wastes storage area that could be used to store data. Secondly, the sector marks which are stored on the data surface are susceptible to media defects. Either a defect on a storage surface can masquerade as a false sector mark or a true sector mark will not be readable because of a media defect. Either one of those scenarios causes problems in locating and retrieving data.
Accordingly, a principal object of the present invention is to overcome the problems encountered by the use of soft sectoring, or storing sector marks directly on the data surface. A further object of the present invention is to provide a system wherein the head may change zones and the system quickly ascertains the sector start locations in the new zone.